Electron multipliers



J. D. MoGEE ELECTRON MULTIPLIERS Filed March 25, 1952 July l, 1958Uw///////////| l -NN v////////// oPAQuE connus 2 on DARK @Lass HQRQIMUZ/2,51

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FIC-L21 /fvenr JAMES DWYER MCGEE i United States Patent O ELECTRONMULTIPLIERS .lames Dwyer McGee, Ealing, London, England, assigner toElectric & Musical Industries Limited, Hayes, England, acompany ofGreatBritain Application March 25, 1952, Serial No. 278,347

Claims priority, application Great ,Britain March 27, 1951 S Claims.`*(Cl. 313-95) This invention relates to electron multipliers of thetype wherein one or `more `secondary electron emissive dynode electrodesare disposed within an envelope within which is also disposed aphoto-electric .cathode and an anode, the .arrangement being such thatthe 'incidence of light on the photo-cathode causes electrons to be emitted therefrom and the impact of said electrons on the `dynode or dynodescauses secondary electrons to be emitted in greater number than theprimary lelectrons whereby the electron current collected by the anodeis greater than the electron current proceeding from the cathode. Insuch multipliers the envelope is normally made of glass and it issometimes found that spurious signals are set up due to the incidence onthe cathode of glow light generated in the vicinity of the anode andpassing along the envelope by internal reection in the wall thereof and/or between said wall and said device or devices. In multipliers of thetype referred to moreover, the dynode electrodes are sometimes supportedby means of glass rods arranged parallel to the axis of the envelope andspurious eiiects may also be caused by light transmitted to the cathodevia said rods from the anode end of the tube.

The object of the invention is to provide means for reducing theseundesirable effects.

According to the invention, an electron multiplier comprises within anevacuated glass walled envelope a photoelectric cathode, at least onedynode electrode and an electron collecting anode, said envelope havinga portion disposed between said cathode and said anode including lightattenuating means interrupting the transmission of light within saidenvelope wall making multiple internal reflections therein.

In order that the invention may be more fully understood the same willnow be described with reference to the accompanying drawings which showin Figures 1, 2 and 3 various forms of the invention applied to electronmultipliers of the above-mentioned type.

The electron multiplier tube shown in Figure 1 is provided with asubstantially cylindrical glass envelope 1 one end of which is closed bya substantially ilat transparent glass end wall 2 on the inner surfaceof which is provided a photo-electric cathode 3. Electrical connectionto the cathode 3 is arranged by means of a contact member 4 hermeticallysealed between the wall 2 and the end of the envelope 1. The other endof the envelope 1 is closed by means of a contact carrying base 5.

Within the evacuated envelope 1 between the cathode 3 and the base 5there are arranged a number of secondary electron emitting dynodes 6 andan electron collecting anode 7. Each of the dynodes 6 comprises a boxlike structure 8 with the side facing the cathode covered by a wire mesh9, the other side having a number of obliquely disposed louvres 10 eachdynode 6 being provided with flanges 11 whereby a series of them can beassembled as a unitary structure by means of glass rods 12, with thelouvres 10 of adjacent dynodes 6 oppositely directed 2,841,728 PatentedJuly 1, 1958 as shown. In the drawing six dynodes 6 :are shown by way,of example although it `will be understood that the number provided maybe varied for diierent tubes depending upon the degree of amplificationrequired. The anode 7 is supported by an arm 13 sealed through the wallof the envelope 1 and electrically connected to a contact cap 14.

Between the `cathode 3 and the first dynode 6 the envelope `1 is formedwith a re-entrant portion 15, the inner and/or outer surfaces of whichare rendered opaque for example by painting as illustrated by theexternal coating 15a. If desired the glass employed for the portion 15ofthe envelope 1 may be `of dark colour such that it will provide `alight attenuator in this region.

In operation of the device with light incident on the cathode 3 and thedynodes 6 and anode 7 maintained positive with respect to the cathode,velectrons which are emitted by the cathode 3, impinge on the firstdynode 6 producing secondary electrons, which impinge on the next dynodeof .the series and release further secondary electrons and so on, thesecondary electrons emitted by the last dynode 6 of said series beingcollected by the anode 7.

During operation `ofthe device, even though itis highly evacuated,'aglow light is liable to occur in the region of lthe anode 7, the 'lightfrom which, if allowed to reach the cathode 3 gives rise to unwantedemission of electrons. By providing a light barrier such as 15, lightpassing from the region of the anode 7 towards the cathode 3 eitheralong the envelope 1 by internal reiiection or through the glass rods 12is intercepted at least partly and the incidence of said light on thecathode 3 is thereby reduced.

In a modification shown in Figure 2: instead of forming the envelope 1with an annular re-entrant portion 15 the light barrier may be formed byan annular metal ring 16 sealed into the cylindrical wall of theenvelope 1.

Said ring 16 may, for example be made from material known by theregistered trademark Kovar, the envelope 1 being made from two portionseach of a glass capable of being sealed to Kovar and being sealedrespectively to the two sides of the metal ring 16. The ring 16 isarranged so that it projects inwardly to a sutiicient extent tointercept light which would otherwise reach the cathode 3 from theregion of the anode 7 by passing along the tube between the cylindricalwall thereof and the dynodes 6 or along the glass support rods 12. Themetal ring 16 may be connected to the cathode 3 either internally orexternally of the tube. In all other respects the tube is the same asthat previously described in connection with Figure 1 and the componentparts have been given the same reference numbers.

An alternative construction is shown in Figure 3, in which in additionto the light barrier forming part of the envelope there is provided afurther light barrier within the envelope arranged between the cathode 3and the first dynode 6.

Referring to Figure 3 the envelope 1 of the device is of uninterruptedcylindrical form along its length from the contact base 5 to the endwall 2. Supported from the glass rods 12 is a cylindrical member 17arranged so as to substantially enclose the space between the cathode 3and the rst dynode 6. The member 17 is of metal with a diameter justslightly less than the internal diameter of the cylindrical envelope 1.One end of the member 17 is arranged to be adjacent the surface of thecathode 3 the other end being provided with an inwardly directedcircular flange 18 which has a circular aperture 19 to allow electronsfrom the cathode 3 to pass to the lrst anode 6. The ends of the rods 12which pass through the apertures in the flange 18 are covered by meansof cup shaped eyelets 20 each of which is welded to the flange 18.

The member 17 is maintained at the potential of cathode 3 by means of aninternal connecting strap 21 and in operation of the device, theelectrostatic eld conditions set up by the inclusion of said member 17between the cathode 3 and the first dynode 6 is such that a uniformwithdrawal of electrons from the whole of the surface area of thecathode 3 is obtained.

The member 17 together with the eyelets 20 will effectivelyintercept'light which would otherwise reach the cathode 3 by passingalong the tube between'the internal cylindrical wall thereof and the-dynodes 6, or along the glass rods 12 but will not prevent lightpassing to the cathode by internal reflection within the glassenvelope 1. Therefore in order to complete the light barrier it isnecessary to construct at least a part of the envelope 1 adjacent theend wall 2 of a light attenuating glass as indicated at 22.

What I claim is:

l. An electron multiplier tube comprising an evacuated envelope, aphotoelectric cathode therein, a plurality of dynode electrodes mountedin said envelope on a plurality of light transmitting insulating rodshaving ends facing said cathode, and an anode for collecting theelectrons emitted by said dynodes, said envelope having a substantialportion of its body lying between said cathode and said rod endscomposed of light transmitting glass, and

having an annular light attenuating portion forming an internalprojection extending towards the axis of the envelope and disposedbetween the cathode and said ends of said rods.

2. An electron multiplier in accordance with claim 1, in which said rodsare made of glass.

3. An electron multiplier in accordance with claim 1, in which saidannular portion is made of light absorbing glass.

4. An electron multiplier in accordance with claim `1, in which saidlight attenuating portion includes a coating of opaque material. y

5. An electron multiplier tube in accordance with claim 1, in which saidinternal projection comprises an opaque annular disc sealed into theenvelope wall and extending inwardly toward the tube axis a suicientdistance to shield the said rod ends optically from said cathode.

i References Cited in the le of this patent UNITED STATES PATENTS GreatBritain Jan. 2o, 1948

